This invention relates to a fluidized bed reactor in which heat is generated by the combustion of fuel in a fluidized bed, and, more particularly, to a sulfur sorbent feed system for the reactor.
Fluidized bed reactors, combustors, or gasifiers, are well known. In these arrangements, air is passed through a bed of particulate material, including a fossil fuel such as coal and an adsorbent for the sulfur generated as a result of combustion of the coal, to fluidize the bed and to promote the combustion of the fuel at a relatively low temperature. When the heat produced by the fluidized bed is utilized to convert water to steam, such as in a steam generator, the fluidized bed system offers an attractive combination of high heat release, high sulfur adsorption, low nitrogen oxides emissions and fuel flexibility.
The most typical fluidized bed combustion system is commonly referred to as a bubbling fluidized bed in which a bed of particulate materials is supported by an air distribution plate, to which combustion-supporting air is introduced through a plurality of perforations in the plate, causing the material to expand and take on a suspended, or fluidized, state. In the vent the reactor is in the form of a steam generator, the walls of the reactor are formed by a plurality of heat transfer tubes. The heat produced by combustion within the fluidized bed is transferred to a heat exchange medium, such as water, circulating through the tubes. The heat transfer tubes are usually connected to a natural water circulation circuitry, including a steam drum, for separating water from the steam thus formed which is routed to a turbine to generate electricity or to a steam user.
In an effort to extend the improvements in combustion efficiency, pollutant emissions control, and operation turn-down afforded by the bubbling bed, a fluidized bed reactor has been developed utilizing a fast, or circulating, fluidized bed. According to this technique, fluidized bed densities are attained which are well below those of a typical bubbling fluidized bed. The formation of the low density circulating fluidized bed is due to its small particle size and to a high solids throughput, which requires high solids recycle. The velocity range of a circulating fluidized bed is between the solids terminal, or free fall, velocity and a velocity which is a function of the throughput, beyond which the bed would be converted into a pneumatic transport line.
Although the circulating fluidized bed enjoys several operational advantages when compared to the bubbling fluidized bed it is not without problems. For example, the sorbent material introduced into the bed is usually of only one particle size. This limits fuel flexibility and causes excessive flyash and bed chemistry problems resulting in sintering and agglomeration. Also, the use of sorbent material of the same particle size causes relatively slow start-ups and load change capability since the solids inventory and the furnace combustor cannot be adjusted rapidly as demanded by the operational requirements.